A pressure vessel is a large-scale apparatus used for containing gases or liquids at above atmospheric pressure. A high-pressure gas tank (or cylinder) is a type of pressure vessel. Examples of the gases or liquids may be hydrogen, helium, oxygen, nitrogen, natural gas, petroleum gas, etc. to be used in automotive and aerospace industries as a propellant, a carrier gas, a diluent gas, a fuel component, etc. These pressure vessels are subject to extreme variations in pressure and temperature; thus, safety and reliability are of a paramount concern in designing such pressure vessels. Some pressure vessels are made of composite materials, such as carbon fiber impregnated with epoxy resin, wound around a metal liner for reinforcement. While high-performance carbon fibers offer high strength-to-weight ratios, the metal liner prevents gas-leakage and provides sturdiness, typically made of aluminum, titanium, alloy, or stainless steel. For lightweight, high-pressure gas containment, aluminum or aluminum alloy is generally a material of choice.
A metal liner for a high-pressure gas tank may be formed to be axially symmetric in shape, having a body section that is used as a main compartment for the gas containment and is shaped to be a generally cylindrical shell, i.e., a generally hollow cylinder elongated in the axial direction, a neck section that is shaped to be a generally narrow tube having a passage therein for discharging the gas from one end of the cylindrical shell, and a shoulder/dome section formed to connect the cylindrical shell and the narrow tube. A valve and fitting may be attached to the neck section for connecting to an external apparatus. The neck sections may be formed at both ends to provide passages for the gas to/from external apparatuses or the ambient.
Spin forming, also known as metal spinning or flow forming, is a manufacturing process, wherein a workpiece such as a disc or tube of metal is rotated at high speed around its cylindrical axis on a mandrel and formed into an axially symmetric part. Spin forming does not involve removal of material, as in grinding or etching, but plastically deforms the workpiece into a final shape. Spin forming often involves a necking process, also known as reducing or closing, wherein a roller, a spoon or other forming tool is pressed against the outer periphery of the workpiece to gradually deform a predetermined portion to have a smaller diameter as it spins, giving rise to a seamless axially symmetric structure having a gradually changing diameter as going along the axial direction. Drawing or cold drawing is another technique for plastic deformation.
Although spin forming techniques have been around since ancient days, the use of spin forming to form metal liners of pressure vessels, especially for aerospace applications, is relatively new. For example, the U.S. Pat. No. 5,822,838 (Seal et al.) discloses the use of spin forming to form domes, based on titanium alloys, followed by heat treatment and machining to remove oxygen-enriched material; the spin-formed domes are then welded to both ends of a cylindrical body to form the liner of a composite overlapped pressure vessel. In another example, the U.S. Pat. No. 6,886,711 (Sakaguchi et al.) discloses the use of spin forming to form a cylindrical shell, based on aluminum alloy, having at least one end open with the maximum wall thickness and a region with a gradually reduced wall thickness connecting to a cylindrical body having the thinnest wall, followed by necking to form a dome section and a neck section with a gradually increasing wall thickness going from the body section to the necked end.
One advantage of spin forming is that several operations can be performed in one set-up, and thus tooling and production costs are comparatively low. Additionally, plastic deforming processes in general waste a considerably less amount of material than other methods, and can build a part having multiple sections without seams by starting from one piece of material. Without seams, a part can withstand high internal or external pressures. However, one inherent disadvantage of plastic deformation is that microscopic folds, wrinkles and other unevenness may likely occur on the surface, developing local stress and fatigue that can eventually lead to cracks or fractures.
High-quality finish of metal surfaces is often required for various mechanical parts to enhance the quality, reliability and safety. Tight requirements are imposed on the fabrication of high-pressure gas cylinders including metal liners, for example, in aerospace engineering and other high-technology applications. In this regard, comprehensive and effective processing techniques are urgently needed to achieve extremely high-quality finish for key metal parts deployed in these areas.